The invention relates generally to the field of wind turbines, and in particular to an active vibration damping solution for variable speed wind turbines.
Wind turbines are regarded as environmentally safe and relatively inexpensive alternative sources of energy. A wind turbine generally includes a rotor that has multiple blades which transform wind energy into a rotational motion of a drive shaft. The drive shaft is utilized to rotate a rotor of an electrical generator. The turbine rotor is rotationally coupled to the generator by a drive train comprising a gear box. The gear box steps up the relatively low rotational speed of the turbine rotor to a more appropriate speed for the generator to efficiently convert the rotational motion to electrical energy. The electrical energy may then be supplied to a utility grid. Typically, the drive train and the generator are housed in a nacelle mounted atop a tower.
Wind shear on the rotating blades causes periodic angular accelerations and decelerations of the rotor, which in turn induces torque oscillations in the drive train. Generally, the drive train is composed primarily of steel components and therefore, exhibits poor passive damping characteristics. Poor damping causes excessive vibrations that adversely affect the life of the turbine components. This situation necessitates active damping solutions that reduce dynamic loads on the drive train and/or turbine structure vibrations.
Present vibration damping solutions generally utilize generator demand torque as an active damping input. Fixed speed wind turbines use induction generators, which have a linear torque-slip curve in the operating region of interest. The generator demand torque produced by such machines is directly proportional to a generator speed. This operational characteristic of induction generators naturally aids in damping oscillations in the drive train. However, active damper designs are necessitated in the case of variable speed wind turbines, which use doubly-fed induction drives. In such machines, the torque demand is no longer restricted to being proportional to slip, resulting in a loss of damping performance. In the past, drive train damping solutions for variable-speed wind turbines have been based on two or three mass lumped parameter descriptions of the dynamics of the drive train. These designs use generator speed feedback as a damper input to damp oscillations induced by drive train resonance by presuming the resonant frequencies of the drive train.
However, such designs often turn out to be inadequate due to one or more of the following reasons. First, since turbines are often configured using components from different vendors, it is difficult to obtain accurate estimates of resonance frequencies of the drive train. This results in sub-optimal operation across various turbine configurations. Secondly, current drive train damping solutions do not mitigate tower side-to-side oscillations, which coupled to the torque oscillations of the drive train.
Accordingly, there is a need for an active damping solution for variable speed wind turbines for mitigating dynamic loads on drive train as well as on the tower, while providing the ability to adapt to different turbine configurations.